This invention pertains to methods and systems for operating a cardiac rhythm management device. In particular, the invention relates to defining and managing periods during which sensing channels of the device are rendered refractory.
Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are fixed or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart""s inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate. Pacing therapy may also be applied in order to treat cardiac rhythms that are too fast, termed anti-tachycardia pacing. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality, regardless of any other functions it may perform such as cardioversion or defibrillation.)
Also included within the concept of cardiac rhythm is the degree to which the heart chambers contract in a coordinated manner during a cardiac cycle to result in the efficient pumping of blood. The heart has specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation (i.e., depolarization) throughout the myocardium. These pathways conduct excitatory impulses from the sino-atrial node to the atrial myocardium, to the atrio-ventricular node, and thence to the ventricular myocardium to result in a coordinated contraction of both atria and both ventricles. This both synchronizes the contractions of the muscle fibers of each chamber and synchronizes the contraction of each atrium or ventricle with the contralateral atrium or ventricle. Without the synchronization afforded by the normally functioning specialized conduction pathways, the heart""s pumping efficiency is greatly diminished. Patients who exhibit pathology of these conduction pathways, such as bundle branch blocks, can thus suffer compromised cardiac output.
Patients with conventional pacemakers can also have compromised cardiac output because artificial pacing with an electrode fixed into an area of the myocardium does not take advantage of the above-described specialized conduction system. This is because the specialized conduction system can only be entered by impulses emanating from the sino-atrial or atrio-ventricular nodes. The spread of excitation from a single pacing site must proceed only via the much slower conducting muscle fibers of either the atria or the ventricles, resulting in the part of the myocardium stimulated by the pacing electrode contracting well before parts of the chamber located more distally to the electrode, including the myocardium of the chamber contralateral to the pacing site. Although the pumping efficiency of the heart is somewhat reduced from the optimum, most patients can still maintain more than adequate cardiac output with artificial pacing.
Heart failure is clinical syndrome in which an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet the metabolic demand of peripheral tissues and is usually referred to as congestive heart failure (CHF) due to the accompanying venous and pulmonary congestion. CHF can be due to a variety of etiologies with ischemic heart disease being the most common. Some CHF patients suffer from some degree of AV block or are chronotropically deficient such that their cardiac output can be improved with conventional bradycardia pacing. Such pacing, however, may result in some degree of uncoordination in atrial and/or ventricular contractions due to the way in which pacing excitation is spread throughout the myocardium as described above. The resulting diminishment in cardiac output may be significant in a CHF patient whose cardiac output is already compromised. Intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) are also commonly found in CHF patients. In order to treat these problems, cardiac rhythm management devices have been developed which provide pacing stimulation to one or more heart chambers in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy.
In conventional pacemakers with sensing channels for sensing one or more heart chambers, the ventricular and/or atrial sensing channels are rendered refractory following certain events, such that certain sensed events are ignored for the duration of the period. Sensing channels are rendered refractory both in order to prevent reentry into the system of an output pacing pulse (in which case the sensing amplifiers are blanked) and to prevent the misinterpretation of input data by the sensing of after potentials or by crosstalk between sensing channels. Cardiac resynchronization therapy may involve pacing both atria, both ventricles, or a heart chamber at one or more pacing sites based upon senses from another site. In order to control the pacing and avoid pacing a chamber or site in the presence of intrinsic activity, sensing channels should be provided for each chamber or site. Sensing both ventricles or both atria, however, requires that refractory periods be managed differently from the situation where only one atria or one ventricle is sensed.
The present invention is a system and method for managing the refractory periods of sensing channels in a cardiac rhythm management device in which both ventricles, both atria, and/or multiple sites in the same heart chamber are sensed. Such sensing configurations can most usefully be employed in delivering cardiac resychronization therapy. The refractory periods for the sensing channels are managed in a manner such that misinterpretation of sense signals is avoided while still allowing the device to pace one or more chambers safely and at the appropriate times.
In an exemplary cardiac resynchronization pacing configuration, heart chambers designated as a rate chamber and a synchronized chamber are sensed through separate channels, and at least one chamber is paced upon expiration of an escape interval without receipt of a rate chamber sense signal. In accordance with the invention, each sensing channel is rendered refractory for a separately selected pacing refractory period after a pacing event, and a sensing channel is rendered refractory for a selected sensing refractory period after receipt of a sense signal from that channel while leaving the other channel non-refractory.
In a particular embodiment of the invention, the rate and synchronized chambers are the right and left ventricles, respectively, and an atrial sensing and pacing channel is also present. The atrial sensing channel may be rendered refractory for selected intervals following an atrial sense, an atrial pace, a pace of either ventricle, or a right ventricular sense signal.